It is a failure while fetching data - that is, before applying any changes - so your system is in no danger.

I would thing that the update command has verbose flags, that would tell you what file failed, at least. Hmm, checked, and it doesn't! Blind Alley there.

No, I just had to read the documentation - that is, the file itself.
-v is for debug levels. Your options are debug, stats, and nostats.
So, try it again with -v debug, and let us know what happens. (You will get screen after screen of nonsense: Maybe use script(1) to save the output for later perusal.

__________________The only dumb question is a question not asked.
The only dumb answer is an answer not given.

This happens sometimes when a lot of patches need to be downloaded. It happens with portsnap sometimes as well when you need, like, 2000+ patches. It may depend on a busy proxy server in the middle, or a congested connection. Repeating the command will usually get you all the patches eventually. With portsnap, I usually revert to a full 'fetch extract'.